Ah, ok the cover/splash plate between block and atx casing.....ok.
Are you sure you have both halves tighly bolted together...
The only part that could rattle is the bottom half...is it bent over and touching the oil pan, etc. If so, bending it back with pliers, etc. should halp. Also is the bottom half secured to the atx. If I remember correctly there should be some bolts to bolt it into the lower half of the atx.
Bob